In this study a thick hexagonal GaN layer has been grown on a (110) single crystalline diamond substrate utilising two different deposition techniques. Using an AlN nucleation layer, metal-organic chemical vapour deposition (MOCVD) has been used to deposit an initial GaN layer on a (I 10) single crystal diamond substrate. The layer consists of closely packed GaN grains with a thickness of approximately 2.5 mum and with different orientations with respect to the substrate. Low temperature photoluminescence indicates a poor optical quality of the layer due to poor structural properties and/or a high incorporation of impurities. This layer was used as a template in a hydride vapour phase epitaxy (HVPE) growth experiment. As a result of this, the GaN grain size has increased enormously and the layer consists of large, hexagonal shaped pillars with a diameter of approximately 50 mum and a height of more than 100 mum protruding from a polycrystalline background having a more uniform thickness. PL spectra of this film show a strongly increased intensity of the exciton related emissions when compared to the MOCVD deposited film. X-Ray diffraction analyses revealed that the dominant orientation of the GaN crystallites perpendicular to the substrate changed from [001] for the thin MOCVD film to [112] for the HVPE layer. (C) 2003 Elsevier B.V. All rights reserved.